In electrophotographic copiers of Carlson's type, the surface of a photoreceptor is charged an electrostatic latent image is formed thereon by exposing to light and is then developed with toner and the resulted visible image is transferred to a sheet of paper or the like and then fixed. At the same time, the photoreceptor is made ready for long, repetitive use after removing adhered toner, neutralizing the static, and cleaning the surface thereof.
Such electrophotographic photoreceptors are therefore required to have excellent electric charging properties and excellent electrophotographic characteristics such as an excellent sensitivity, a properly limited dark-decay and so forth as the matter of course and, in addition, excellent physical properties such as copying durability, abrasion resistance, moisture resistance and so forth in repetitive use, and excellent resistance against environmental impacts such as resistance against ozone generated by a corona discharge, UV rays generated by an exposure to light, and so forth.
For a long time now, an inorganic photoreceptor having a photoreceptive layer principally comprising such an inorganic photoconductive material such as selenium, zinc oxide, cadmium sulfide or the like has popularly been used for an electrophotographic photoreceptor.
In recent years, on the other hand, some researches and developments have actively been attempted to utilize various types of organic photoconductive materials as the materials for the photoreceptive layers of electrophotographic photoreceptors.
For example, an organic photoreceptor having a photoreceptive layer containing poly-N-vinyl carbazole and 2,4,7-trinitro-9-fluorenone is described in Japanese Patent Examined Publication No. 10496/1975. This photoreceptor does not always satisfy sensitivity and durability. With the purpose of overcoming the above-mentioned defects, some attempts have been made to develop an organic photoreceptor having a high sensitivity and a great durability, in which two materials different from each other are assigned separately to display the charge generation and charge transport functions in the photoreceptive layer, respectively. In this kind of the so-called function-separated type electrophotographic photoreceptors, the materials capable of displaying the above-mentioned functions may be selected from a wide range of materials. It is therefore comparatively easier to prepare an electrophotographic photoreceptors having any desirable properties.
The efforts have gone into the development of an electrophotographic photoreceptor having an excellent electrophotographic properties and a layer strength, in such a manner that a low molecular weight organic compound is used for a charge transportation material, and a material capable of generating a desired charge and a high molecular binder are used in combination.
The above-mentioned high molecular binders include, for example, polycarbonates which are excellent in the properties of electric charge, repetitive use, and so forth. ##STR2##
These polycarbonates have such a structure that two methyl groups are symmetrically bonded to a carbon atom being in the center of bisphenol A. Upon examination, it was found that the above-mentioned polycarbonates have the following defects:
(1) Mechanical strength, especially, scratch and abrasion resistance are so unsatisfactory that any organic photoreceptors may not be made more durable.
(2) Compatability with a carrier transportation material (hereinafter sometimes abbreviated to CTM) is so poor that CTM crystal deposition is apt to be produced, so that there may be some instances where a layer may be cracked.
(3) When using the above-mentioned polycarbonates having been highly concentrated in a dip-coating method for example, the gelation of a coating solution is liable to be produced due to the crystallization of the polycarbonates, so that the life of the coating solution will be shortened.
(4) When forming a layer, the gel-like material forms a protrusion on the surface of the layer and then the protrusion has a comet produced on the layer so as to lower the yield, or, when using a photoreceptor, toner adheres to the surface of the protrusion and remains uncleaned as it is, so that an image defect is liable to be produced by the so-called toner-filming.
The reason why the above-mentioned polycarbonates are liable to be crystallized is seemed to be that, in the above-mentioned polycarbonates, the groups bonded to the carbon atom being in the center of bisphenol A are comprised of the lowest methyl groups which produce a high grade molecular chain orientation.
For solving the problems derived from the polycarbonates liable to be crystallized, Japanese Patent O.P.I. Publication Nos. 60-172004/1985 and 60-172045/1985 have proposed to use a non-crystallizing polycarbonate in a photoreceptive layer. With such non-crystallizing polycarbonates, the above-mentioned problems may be solved and a photoreceptor having excellent mechanical strength and scratch and abrasion resistance may be provided.
However, after the inventors repeated examination, they have resultingly found that every photoreceptor using the above-mentioned non-crystallizing polycarbonates deteriorates the electric property and increase the residual potential, when using repeatedly.
As described above, it has been difficult to provide excellent electrophotographic photoreceptors capable of satisfying the requirements such as layer casting property, mechanical strength and scratch and abrasion resistance as well as electric property in repetitive use. Accordingly, there have so far been the demands for a photoreceptor capable of satisfying the above-mentioned every requirement altogether.